In the same way an autopilot frees a pilot from the manual operations of flying, a navigation system relieves you of many manual operations required to direct the aircraft. When sensors are tied into a navigation system, the system automatically uses their data to compute present position for the navigator. During a flight from the United States to a foreign country, the aircraft may pass over areas of land, water, and icecaps. You may have to deal with conditions of overcast, undercast, day, night, altitude changes, turn points, and air traffic requirements. To handle these conditions at high speeds more effectively, the navigator uses a navigation system.
Types of Systems
Navigation systems can be classified according to many criteria. Systems can be classified by capability, such as visual flight rules (VFR)-only or all-weather. They can be classified as either self-contained or externally-referenced. Each system has advantages and disadvantages, but this discussion is confined to self-contained and externally referenced systems.
Self-Contained Navigation Systems
Self-contained systems (radar, celestial, INS, etc.) are complete in that they do not depend upon externally transmitted data.
Externally-Referenced Navigation Systems
Externally-referenced aids (GPS, NAVAIDs, etc.) include all aids that depend upon transmission of energy or information from an external source to the aircraft. While externally referenced aids have enormous installation and operating costs to the system administrator, they have much lower equipment and maintenance costs to the user.
The Ideal System
Every navigation system has certain advantages and disadvantages. A particular navigation system is selected for use in an aircraft when its advantages outweigh its disadvantages. In some cases, several components are included in a system to provide adequate, redundant information for all possible flight situations. The ultimate navigation system should have the following characteristics:
- Groundplot DR information—the system must indicate the position and velocity relative to the ground.
- Global coverage—capable of positioning and steering the aircraft accurately and reliably any place in the world.
- Self-contained—must not rely on ground or space transmissions of any kind.
- Flexible—works well despite unplanned deviations. The system must work well at all altitudes and speeds.
The navigational system consists of three parts:
- The computer or central processing unit (CPU)
- Data-gathering sensors, such as astrotrackers, GPS, ground-mapping radar, or NAVAIDS
- An operator input/output (I/O) interface
The CPU takes in all available data and converts it into usable navigation information. Control panels or computer keyboards allow the operator to control and make inputs to the computer. Data is displayed for the operator on display panels, radar screens, or computer screens. Additional hardware components could include terrain following radar or television cameras.
Most navigation systems are hybrids of the two basic computer types: analog and digital.
Analog computers are more specific in design and function than digital computers. While analog computers process vast amounts of similar data, they are not very flexible and cannot be used for multiple purposes. Radar scan converters efficiently process collected radar signals into video images. Video processors collect and process images into video displays. Other examples of analog computers are terrainavoidance computers and terrain-following computers.
Digital computers are lighter and more compact than analog computers. Hand-held calculators and laptop computers are two examples of the miniaturization possible with digital computers. You can put a great deal of computing power and capability into a small box; the biggest limitation is increased cost. An analog radar scan converter is very efficient at processing radar data, but it cannot be used for other applications. On the other hand, digital computers can be loaded with navigation software, aerial delivery software, and diagnostic programs. These computers can mathematically manipulate data in any way imaginable, because they deal strictly with digital information. The output from digital computers may need to be converted into an analog format for most efficient use by the navigator; however, the digital computer cannot do that. It can display the digital data in an approximation of analog data. While a digital computer can perform any mathematical function, it must first be programmed for that function. Inflight reprogramming is not generally possible.
Many types of sensors are used for inputs to navigation systems.
The use of astrotrackers has decreased; however, they are still excellent sources of position information. They automatically track celestial bodies and compute position information using celestial techniques. They are passive but require clear skies.
The Doppler radar measures groundspeed and drift. These two data inputs can be put to several uses in the computer system. Doppler groundspeed is used to determine distance to update the aircraft position. Drift can be used to compute winds and aircraft track. Doppler outputs can be used in platform leveling and verifying inertial groundspeed in an INS. Doppler radar is an essential part of many navigation computer systems.
The gyro-stabilized magnetic heading source is corrected to true heading with the local magnetic variation. This can be applied manually or automatically from a database in the computer. Magnetic or true course can be calculated by applying doppler or inertial drift.